74 
MR. HARRIS ON THE TRANSIENT MAGNETIC STATE 
bodies : at least for such distances as can be conveniently resorted to with an 
intervening screen. In order to investigate this, I resorted to the machine 
already described (6) and represented in fig. 3 & 8. By a few previous trials 
such weights were found as might in a short time impart to the rotating disc 
the respective velocities of 178.5, 357 and 714 revolutions in a minute ; which 
numbers are to each other as the numbers 1, 2, 4 : and by means of the index 
and graduated circle x, fig. 3 & 8, the deflections of a magnet could be taken 
when these velocities were attained. This point was determined by means of 
a valuable chronometer of a peculiar description which my friend Lieut.-Col. 
H. -Smith, F.R.S. was so good as to lend me for the purpose ; it can be set 
going, and stopped again at pleasure, and is capable of registering an observa- 
tion to the cVth part of a second. As the graduated circle is divided into 
twenty parts, it is presumed that the rate of motion can in this way be ascer- 
tained with sufficient accuracy ; and thus any little acceleration caused by the 
descending weight is not of consequence. 
12. A magnetic bar being suspended as in fig. 3, was in the first place 
accurately adjusted at the point of contact to the plane of the body intended 
to be put in motion, and which consisted of a flat, ring of copper of an inch 
wide, and about 0.05 of an inch thick. The bar was then raised from the ring 
through a distance equal to five turns of a micrometer-screw at D, each turn of 
the screw being equal to the ^oth of an inch : the screen s was then interposed, 
and the whole covered by a receiver, and exhausted. 
(>?,) The machine being set in motion, the deviation of the bar amounted to 
24° when the velocity was 357 revolutions in a minute : on increasing the velo- 
city to 714 revolutions in a minute, or double the former, it amounted to 56°. 
The exhaustion in these experiments was carried to within 0.5 of an inch. 
Taking the sines of the angles of deviation as a measure of the force urging the 
bar, we have the respective numbers .829038 and .406737, which are very nearly 
in the same ratio as the respective velocities ; that is to say as 2 : 1. 
(o) The bar being adjusted to within a distance of the ring equal to four 
turns of the screw, the deviation of the needle amounted to 38° when the velo- 
city was 357 revolutions in a minute: on raising the bar by four additional 
turns of the screw, the deviation decreased to 9°. Taking the sines of these 
angles as before, we have for the corresponding distances, the numbers .615661 
